Transient cerebral ischemia depolarizes neurons, increases extracellular glutamate and intracellular calcium, and changes synaptic efficacy in the post-ischemic phase. The effects of transient ischemia are equivalent to those produced by strong electrical stimulation within the compromised area. We present evidence that transient ischemia induces marked biochemical and ultrastructural changes in postsynaptic densities (PSDs). Transient ischemia was induced in rats by bilateral ligation of the carotid arteries for 15 min followed by 4 hr of reperfusion. In these animals, the yield of PSDs as measured in isolated fractions of cortex was increased by a factor of 2.4910.23 over controls. The protein composition of the reperfused PSDs was reconstituted as demonstrated by Coomasie blue staining and Western Blots. The most remarkable changes were an accumulation of protein kinases in reperfused PSDs including p38-kinase, JNK1, gp145trkB, CaM-kinase II, protein kinase C and tyros ine kinases. The phosphorylation states of PSD proteins were also markedly increased following reperfusion. These biochemical alterations were consistent with ultrastructural modifications observed in PSDs prepared for electron microscopy. PSDs isolated from post-ischemic animals were longer and thicker than those from controls. PSD structure was also dramatically altered in tissue sections from the cortex of reperfused animals stained with phosphotungstic acid to selectively stain PSDs. PSDs were noticeably thicker, more irregular in shape and less compact than in controls. We conclude that docking of signal transduction molecules in PSDs and phosphorylation of PSD proteins are important contributors to changes in synaptic efficacy following stimulation. This work was published in Hu et al., J. Neurosci., 18: 625-633, 1998. In the past year, we performed a more detailed quantitative analysis of PSD changes in CA1 and dentate gyrus in ischemic hippocampus. Using thick sections and IVEM, we performed tomographic reconstructions of synapses in the hippocampus of ischemic and control rats at different time points of reperfusion. These reconstructions clearly showed that synapses in area CA1 were more loosely configured in the ischemic brain than in control brains. The 3D images suggest that synapses in CA1 are undergoing degenerative changes prior to obvious cell death. This work was presented at the Annual Society for Neuroscience meeting and a manuscript has been submitted to the Journal of Neuroscience.